Paleopressure during Hydrocarbon Charging and Its Evolution in the Funing Formation of the Gaoyou Sag, Subei Basin, Eastern China

Abstract

Abnormally high pressures are currently limited and locally developed in the Funing Formation of the Gaoyou Sag, Subei Basin, eastern China, but the paleopressure and its evolutionary history remain unclear. Based on the determination of hydrocarbon charging periods by performing systematic fluid inclusion analysis on sixteen core samples from the Funing Formation, thermodynamic modeling with fluid inclusion data was adopted to reconstruct the paleopressure and redisplay its evolutionary history throughout geological time. Results showed that the Funing Formation experienced two episodes of hydrocarbon charging periods. Episode 1 occurred with the charging of lower maturity oils in the period from 52.8 Ma to 49.5 Ma, which was recorded by yellow-fluorescing oil inclusions. Episode 2 happened with the charging of higher maturity oils in the period from 47.0 Ma to 37.0 Ma, which was characterized by blue-fluorescing oil inclusions. Each episode was an abnormally high-pressured hydrocarbon charging process. The pressure coefficient of Episode 1 reached as high as 1.44, while that of Episode 2 reached as high as 1.40. The current formation pressure is the evolutionary result of paleopressure after a process of rapid increasing and decreasing and slow increasing and is not as high as what it reached during the hydrocarbon charging periods. This work is valuable for the exploration of conventional clastic oil reservoirs and unconventional shale oils in the Funing Formation.